Terahertz photoconductive antennas (THz-PCAs) are used to generate or receive terahertz (THz) signals such as broadband terahertz pulses and narrowband terahertz continuous waves. After being demonstrated as practical THz sources and detectors, THz-PCAs have been the subject of a significant amount of scientific and industrial reports investigating their applications as terahertz wave transmitters and receivers.
Narrowband terahertz continuous waves can be generated using two continuous-wave laser beams that are selected to provide a frequency difference in the THz range. The laser beams are combined inside an optical fiber or overlapped in free-space and subsequently mixed in a photomixer (e.g. a photo-absorbing medium) to generate a THz wave. In some cases, THz waves having a frequency line width of as low as a few KHz can be generated using this technique.
Broadband terahertz pulses can be generated by exciting a THz-PCA with a femto-second short pulse laser. For example, when using a femto-second laser with an optical pulse duration of about 100-fs it is possible to generate THz pulses having a frequency content of up to about 5-THz, and an average power of a few micro-watts.
In addition to generating THz waves, THz-PCAs are also capable of detecting THz waves. For example, optoelectronic techniques can be used to detect and extract both amplitude and phase information of a THz wave incident on an optically gated THz-PCA. In this case, a gating laser beam is applied using a short pulse laser, or a pair of continuous-wave lasers having a frequency difference selected to be equal to the frequency of the incident terahertz wave. The THz wave and gating laser beam applied to the THz-PCA generate a photocurrent, which is generally proportional to the amplitude of the incident terahertz wave.
In conventional THz systems, one side of the THz-PCA is excited by one or more laser beams that pass through free-space to a focusing lens. Furthermore, a hyper-hemispherical silicon lens is positioned on the other side of the THz-PCA for collimating the THz signal being generated or detected. In practice, the THz-PCA and hyper-hemispherical silicon lens are mounted on two separate X-Y translation stages for maintaining optical alignment of the silicon lens, THz-PCA, and focusing lens. The X-Y translation stage typically includes a base and a platform slidably mounted to the base for supporting and moving the THz-PCA or the silicon lens. Unfortunately, these X-Y translations stages are cumbersome to operate and increase overall system complexity, thus making it difficult to calibrate and use the THz-PCA.
Accordingly, there is a need for a new apparatus for transmitting or receiving terahertz waves.